Lithium-soap greases are widely used as all-purpose lubricants in industrial machines, cars, railroad trains, and so forth. These greases are usually prepared by adding hydroxy-fatty acid, dicarboxylic acid, and lithium hydroxide to a mineral oil component, and then reacting them by heating. This reaction, however, inevitably becomes complicated, since the rates of reaction of hydroxy-fatty acid and lithium hydroxide are different from those of dicarboxylic acid and lithium hydroxide and, furthermore, the solubilities of the resulting lithium soap or salt in the mineral oil are different from each other. Moreover, it is necessary to apply a heating operation several times during the process of the production of the lithium soap (see, for example, U.S. Pat. Nos. 3,791,973 and 3,681,242). This is apparently disadvantageous from a viewpoint of energy consumption.